1. Field of the Invention
This invention relates to a method for storing an image signal, wherein an image signal, which has been generated by scanning an image in a main scanning direction and a sub-scanning direction and is made up of a series of image signal components, is stored on a storage medium, such as an optical disk.
2. Description of the Prior Art
Storing a plurality of images, picking a certain image out of the plurality of the images, and using it have heretofore been carried out in various fields. For example, at medical facilities, such as hospitals, many kinds of medical images are utilized for medical treatment or research. Most of the medical images are X-ray images. In recent years, many computed tomography (CT) images, magnetic resonance (MR) images, and the like, are also utilized.
The medical images should be stored to permit investigation of changes in diseases or injuries of patients, and it is legally stipulated that the medical images must be stored for a predetermined period. Therefore, in hospitals, or the like, the number of stored medical images increases daily.
The medical images have heretofore been stored in the form of hard copies. However, storage as hard copies requires a large storage space and burdensome operations for control and retrieval of the medical images in hospitals, or the like.
Accordingly, in recent years, an image filing apparatus has been proposed in which images, such as medical images, are retrievably stored (i.e. filed) as image signals on a storage medium. In cases where the medical images are filed in this manner on the storage medium, the space requirement and the burden of image storage can be reduced, and the images can be retrieved easily and quickly.
In the image filing apparatus, image signals are stored on a storage medium, such as an optical disk, which has a very large storage capacity. However, even an image signal representing a single image requires a considerable storage capacity. Therefore, in general, image signals are compressed, and the resulting compressed image signals are stored on the storage medium.
Various processes have heretofore been proposed to compress image signals. One example of such processes is the so-called interpolation encoding process. With the interpolation encoding process, image signal components of an image signal, which represent a plurality of picture elements in an image, are sampled at appropriate sampling intervals, e.g. alternately. The image signal components, which have thus been sampled, are employed as main image signal components. As for each of the remaining picture elements other than the picture elements, which are represented by the main image signal components, a calculation is made to find the difference between a value, which has been interpolated from the main image signal components representing the picture elements neighboring with each remaining picture element, and the actual value of the image signal component representing each said remaining picture element. In this manner, interpolated image signal components representing the differences, which have thus been calculated, are generated for the remaining picture elements. Thereafter, the main image signal components and the interpolated image signal components are subjected to compression processing, and are stored in the compressed form on a storage medium.
Also, retrieval information giving specifics about each image is entered into the image filing apparatus together with the image signal representing the image. A data base is constructed from the retrieval information corresponding to a plurality of image signals and is used during the retrieval of the image signals.
When the original image signal representing the image is to be restored, the main image signal components, which have been compressed, are subjected to decompression processing, which corresponds to the reversed operation of the compression processing. In this manner, the main image signal components, which have not been compressed, are restored. Also, the interpolated image signal components, which have been compressed, are subjected to decompression processing, which corresponds to the reversed operation of the compression processing. Thereafter, the values of the image signal components representing the picture elements other than the picture elements, which are represented by the main image signal components, are calculated from the values of the interpolated image signal components, which have thus been decompressed, and the values of the restored main image signal components. The image signal components representing the picture elements other than the picture elements, which are represented by the main image signal components, are thus restored and then combined with the restored main image signal components. In this manner, the original image signal representing the image is restored.
In the image filing apparatus described above, in the course of retrieving and reading a compressed image signal from the storage medium, such as an optical disk, if the storage medium has a defective part, the compressed image signal components (the main image signal components or the interpolated image signal components, which have been compressed) will not be read out normally. As a result, part of the compressed image signal components, which have thus been read out of the storage medium, will be destroyed.
As described above, with the interpolation encoding technique, an image signal representing a single image is divided into the main image signal components and the interpolated image signal components, and the main image signal components and the interpolated image signal components are then compressed. Therefore, in cases where the compressed image signal components read from the storage medium are ones which have been compressed with the interpolation encoding technique, if part of the compressed image signal components (the main image signal components or the interpolated image signal components, which have been compressed) is destroyed, it will become impossible to find how the main image signal components and the interpolated image signal components are associated with each other. In such cases, the destroyed part of the compressed image signal components and part thereof following the destroyed part cannot be restored.